The present invention is a method for controlling the carbon balance of a silicon smelting furnace. The inventors have discovered that when the concentration of calcium within the silicon formed in the furnace is maintained within a range of about 0.4 to 2.0 weight percent, the calcium yield of the furnace is a rapid and sensitive indicator of the carbon balance of the furnace. More specifically, the inventors have found that a calcium yield of less than about 80 weight percent indicates a negative carbon balance in the furnace, a calcium yield of greater than about 90 weight percent indicates an excess carbon balance in the furnace, and a calcium yield within a range of about 80 weight percent to 90 weight percent indicates a furnace in carbon balance. In a preferred embodiment of the present invention, calcium yield is used in conjunction with a harmonic volts parameter and a electrode consumption measurement to control carbon balance of the furnace.
Elemental silicon is produced by the carbothermic reduction of silicon dioxide (SiO,) according to the overall reaction: EQU SiO.sub.2 +2C.fwdarw.Si+2CO (1)
It is well known to those skilled in the art that this reaction proceeds through a number of intermediate reactions involving the production and reaction of silicon monoxide (SiO) and silicon carbide (SiC). Important intermediate reactions for the purpose of this invention can be summarized as: The reaction of silicon dioxide with carbon to form silicon carbide, EQU SiO.sub.2 +3C.fwdarw.SiC+2CO, (2)
the reaction of silicon monoxide with carbon to form silicon carbide, EQU SiO+2C.fwdarw.SiC+CO, (3)
and the reaction of silicon with carbon to form silicon carbide, EQU Si+C.fwdarw.SiC (4)
As equation (1) illustrates, theory suggests that for a silicon smelting furnace to be in carbon balance two moles of carbon should be added to the furnace per mole of silicon dioxide. This condition is described as 100 percent carbon theory. However, due to process inefficiencies, operation of a silicon smelting furnace does not proceed exclusively according to reaction (1).
If, for example there is insufficient or unreactive carbon, in bulk or locally, to effect reaction (3) a portion of the silicon monoxide will exit the charge bed in the offgas. This situation can occur due to raw materials selection, bed design, and unbalanced stoichiometry in the bed. The loss of silicon monoxide from the furnace results in reduced recovery of elemental silicon. In addition, in the case of insufficient carbon, increased consumption of carbon electrodes used in the furnace can occur. In extreme cases of carbon deficiency, carbon used as furnace lining may be consumed.
Conversely, if too much carbon is present in the furnace, in bulk or locally, reactions (2), (3), and (4) can cause silicon carbide accumulation and reduced silicon production. The accumulated silicon carbide can cause filling of the furnace causing the electrode to be raised out of the proper operating position. In addition, when excess silicon dioxide is added to react with the accumulated silicon carbide, the additional silicon monoxide formed can cause increased electrode consumption and loss of process yield of elemental silicon.
Therefore, it is important for efficient furnace operation that the furnace be kept in carbon balance. However, as a result of the described inefficiencies, carbon balance in a silicon smelting furnace cannot be maintained by merely adding carbon and silicon dioxide to the furnace in a two to one molar ratio, based on carbon theory.
Therefore, it is an objective of the present invention to provide a method where carbon balance of a silicon smelting furnace can be determined within a fairly short time period (e.g. four to eight hours) and adjustments made to carbon balance as needed. The benefits provided by the described method can include reduced electrode consumption, higher process yields, and less furnace shutdowns.
Dosaj et al., U. S. Pat. No. 5,174,982, issued Dec. 29, 1992, describes a method for assessing the carbon balance of a silicon smelting furnace by measuring the amount of carbon monoxide evolved in offgas exiting the furnace.
Halvorsen, U. S. Pat. No. 4,539,194, issued Sep. 3, 1985, describes adding one to ten percent calcium to silicon to facilitated subsequent purification steps.
Dosaj et al., U. S. Pat. No. 4,798,659, issued Jan. 17, 1989, describe an improvement to a process for the preparation of silicon from the reduction of silicon dioxide with a solid carbonaceous reducing agent. The improvement comprises feeding calcium compounds into the reaction zone of a silicon furnace and controlling and maintaining a desired calcium level in the reaction zone of the silicon furnace. Dosaj et al. teach that the calcium compounds may be fed to the silicon furnace as a constituent of either the silicon dioxide or solid carbonaceous reducing agent feeds, as a separate feed, or as a combination of two or more of these feeds.
The cited art does not recognize that calcium yield of a silicon smelting furnace can be used as an indicator of carbon balance within the furnace, thus allowing improved control of the carbon balance and resultant improvements in the furnace performance.